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ARCH_PRCTL(2)			    Linux Programmer's Manual			    ARCH_PRCTL(2)

NAME
       arch_prctl - set architecture-specific thread state

SYNOPSIS
       #include <asm/prctl.h>
       #include <sys/prctl.h>

       int arch_prctl(int code, unsigned long addr);
       int arch_prctl(int code, unsigned long *addr);

DESCRIPTION
       The  arch_prctl()  function  sets  architecture-specific  process  or  thread state.  code
       selects a subfunction and passes argument addr to it; addr is  interpreted  as  either  an
       unsigned  long  for  the  "set" operations, or as an unsigned long *, for the "get" opera-
       tions.

       Subfunctions for x86-64 are:

       ARCH_SET_FS
	      Set the 64-bit base for the FS register to addr.

       ARCH_GET_FS
	      Return the 64-bit base value for the FS register	of  the  current  thread  in  the
	      unsigned long pointed to by addr.

       ARCH_SET_GS
	      Set the 64-bit base for the GS register to addr.

       ARCH_GET_GS
	      Return  the  64-bit  base  value	for  the GS register of the current thread in the
	      unsigned long pointed to by addr.

RETURN VALUE
       On success, arch_prctl() returns 0; on error, -1 is returned, and errno is set to indicate
       the error.

ERRORS
       EFAULT addr points to an unmapped address or is outside the process address space.

       EINVAL code is not a valid subcommand.

       EPERM  addr is outside the process address space.

CONFORMING TO
       arch_prctl() is a Linux/x86-64 extension and should not be used in programs intended to be
       portable.

NOTES
       arch_prctl() is supported only on Linux/x86-64 for 64-bit programs currently.

       The 64-bit base changes when a new 32-bit segment selector is loaded.

       ARCH_SET_GS is disabled in some kernels.

       Context switches for 64-bit segment bases are rather expensive.	It may be a faster alter-
       native  to  set	a  32-bit  base  using	a segment selector by setting up an LDT with mod-
       ify_ldt(2)  or  using  the  set_thread_area(2)  system  call  in  kernel  2.5  or   later.
       arch_prctl()  is  needed only when you want to set bases that are larger than 4GB.  Memory
       in the first 2GB of address space can be allocated by using  mmap(2)  with  the	MAP_32BIT
       flag.

       As  of  version 2.7, glibc provides no prototype for arch_prctl().  You have to declare it
       yourself for now.  This may be fixed in future glibc versions.

       FS may be already used by the threading library.

SEE ALSO
       mmap(2), modify_ldt(2), prctl(2), set_thread_area(2)

       AMD X86-64 Programmer's manual

COLOPHON
       This page is part of release 3.53 of the Linux man-pages project.  A  description  of  the
       project,     and    information	  about    reporting	bugs,	 can	be    found    at
       http://www.kernel.org/doc/man-pages/.

Linux					    2007-12-26				    ARCH_PRCTL(2)
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